instrumental techniques applied to mineralogy and geochemistry

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Analytical techniques applied to fluid inclusion studies: basics and applications 141 liquid in the cavity (eutectic temperature). This temperature is characteristic of each system and, therefore, allows us to associate the composition of the fluid inclusion with a chemical system. Table 2 summarizes some of the most common chemical systems found in natural fluids as well as their eutectic temperatures. In regard to the gaseous phase, the first change that should be observed during heating is the melting of CO 2 -or other gases that could be present in the cavity-. Table 3 includes some microthermometric characteristics of the most frequently occurring volatiles in nature. Later, other phase transitions may be observed in the liquid phase, in the vapour phase or at the interphase region (melting of the last crystal of ice, melting of hydrohalite or other salt hydrates, melting of clathrates, homogenization of CO 2 , etc.). Using the temperatures observed for these phase transitions, and with adequate diagrams, we can estimate the salinity, types and contents of the volatiles, etc. Later, the inclusions are heated from room temperature to the temperature at which homogenization of the cavities occurs. This temperature is known as the homogenization temperature and represents the minimum trapping temperature for the fluid inclusion. To determine the P-T trapping conditions, it is necessary to calculate the isochore corresponding to the fluid inclusion. In a space P-T, this isochore determines a straight line, for which it can be said that: (1) its slope is a function of the density of the fluid inclusion; (2) the homogenization temperature is the minimum value of the aforementioned straight line; (3) the line contains the trapping P-T, although with unknown coordinates until the present time. To determine the trapping P-T, it is necessary to use an independent geothermometer/geobarometer or any of the procedures described in the relevant literature (see for example chapter 7.6 of Shepherd et al., 1985). Using different data input (microthermometric data, micro raman data, cation ratios, volume fraction estimations, etc.), several computer packages make extensive calculations related with fluid inclusions (isochores, bulk density, molar volume, salinity, composition of fluid inclusion, etc.). In this sense, the most popular could be FLINCOR (Brown, 1989), MacFlinCor (Brown & Hagemann, 1994), CLATHRATES (Bakker, 1997) or FLUIDS (Bakker, 2003). See also Bakker & Brown (2003) for more details about these software packages, and the website http://institute.unileoben.ac.at/mineralogie/Bakker/Programs/Computer.html for latest updates.
142 Salvador Morales Chemical system Eutectic temperature H 2 O-NaCl-CaCl 2 H 2 O-CaCl 2 H 2 O-NaCl-FeCl 2 H 2 O-FeCl 2 H 2 O-NaCl-MgCl 2 H 2 O-MgCl 2 H 2 O-NaCl-KCl H 2 O-NaCl H 2 O-KCl -52.0°C -49.5°C -37.0°C -35.0°C -35.0°C -33.6°C -22.9°C -20.8°C -10.6°C TABLE 2. Eutectic temperature for the most frequent naturally occurring salt system (simplified from Shepherd et al., 1985). Species T triple (°C) T critical (°C) Raman frequency (cm -1 ) N 2 -210.0 -147.0 2331 CO 2 -56.6 31.1 1285 CO 2 -56.6 31.1 1388 CH 4 -182.5 -82.1 2917 H 2 S -85.5 100.4 2580 (liquid) H 2 S -85.5 100.4 2590 (in water) H 2 S -85.5 100.4 2611 TABLE 3. Temperature of the triple point, critical temperature and Ramam frequency for common volatiles in fluid inclusions (summarized from Shepherd et al., 1985; Burke, 2001 and Diamond, 2003). Use of other study techniques In the majority of fluid inclusion studies, a microthermometric characterization is sufficient to identify the fluids that could have intervened in the genesis of the fluid inclusions and to characterize their conditions of formation, which is the usual aim of this type of studies.
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